1. Field of the Invention
The present invention relates to orally administered peptide pharmaceuticals where the active compounds include a plurality of amino acids and at least one peptide bond in their molecular structures, and to methods of enhancing bioavailability of such peptide active compounds when administered orally.
2. Description of the Related Art
Numerous human hormones, neurotransmitters, cytokines, growth factors and other important biological compounds have peptides as a substantial part of their molecular structures. Many diseases respond positively to raising the level of these peptide compounds in patients. Therapeutically effective amounts of such biologically relevant peptides may be administered to patients in a variety of ways. However, as discussed further below, preferred oral administration is very difficult with this type of active compound.
Salmon calcitonin, for example, is a peptide hormone which decreases calcium release from bone. When used to treat bone-related diseases and calcium disorders (such as osteoporosis, Paget's disease, hypercalcemia of malignancy, and the like), it has the effect of helping maintain bone density. Many types of calcitonin have been isolated (human calcitonin, salmon calcitonin, eel calcitonin, elkatonin, porcine calcitonin, and chicken calcitonin). There is significant structural non-homology among the various calcitonin types. For example, there is only 50% percent identity between the amino acids making up human calcitonin and those making up salmon calcitonin. Notwithstanding the difference in molecular structure, salmon calcitonin may be used in the human treatment of the calcitonin-responsive diseases discussed above.
Another example of peptide hormone is parathyroid hormone(PTH). PTH is produced by the parathyroid gland and is a major regulator of blood calcium levels. PTH is a polypeptide and synthetic polypeptides may be prepared by the method disclosed by Erickson and Merrifield, The Proteins, Neurath et al, Eds., Academic Press, New York, 1976, page 257, and as modified by the method of Hodges et al (1988), Peptide Research 1, 19, or by Atherton, E. and Sheppard, R. C., Solid Phase Peptide Synthesis, IRL Press, Oxford, 1989.
When serum calcium is reduced to below a normal level, the parathyroid gland releases PTH and the calcium level is increased by resorption of bone calcium, by increased absorption of calcium from the intestine, and by increased renal reabsorption of calcium from nascent urine in the kidney tubules. Although continuously infused low levels of PTH can remove calcium from the bone, the same low doses, when intermittently injected can actually promote bone growth.
Tregear, U.S. Pat. No. 4,086,196, described human PTH analogues and claimed that the first 27 to 34 amino acids are the most effective in terms of the stimulation of adenylyl cyclase in an in vitro cell assay. Rosenblatt, U.S. Pat. No. 4,771,124, disclosed the property of hPTH analogues wherein Trp23 is substituted by amino acids phenylalanine, leucine, norleucine, valine, tyrosine, β-naphthylalanine, or α-naphthylalanine as a PTH antagonist. These modified hPTH analogues also have the 2 and 6 amino terminal acids removed, resulting in loss of most agonist activities when used to treat osteoporosis. These analogues were designed as inhibitors of PTH and PTH-related peptides. The analogues were claimed as possibly useful in the treatment of hypercalcemia associated with some tumors.
Pang et al, WO93/06845, published Apr. 15, 1993, described analogues of hPTH which involve substitutions of Arg25, Lys26, Lys27 with numerous amino acids, including alanine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, histidine, isoleucine, leucine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, or valine. These are claimed to be effective in the treatment of osteoporosis with minimal effects on blood pressure and smooth muscle.
PTH operates through activation of two second messenger systems, Gs-protein activated adenylyl cyclase (AC) and Gq-protein activated phospholipase Cβ. The latter results in a stimulation of membrane-bound protein kinase Cs (PKC) activity. The PKC activity has been shown to require PTH residues 29 to 32 (Jouishomme et al (1994) J. Bone Mineral Res. 9, (1179-1189). It has been established that the increase in bone growth, i.e., that effect which is useful in the treatment of osteoporosis, is coupled to the ability of the peptide sequence to increase AC activity. The native PTH sequence has been shown to have all of these activities. The human hPTH-(1-34) sequence is typically shown as:
Ser Val Ser Glu Ile Gln Leu Met His Asn Leu Gly Lys His Leu Asn Ser Met Glu Arg Val Glu Trp Leu Arg Lys Lys Leu Gln Asp Val His Asn Phe-OH (SEQ ID NO:1).
Various PTH analogues are disclosed in U.S. Pat. Nos. 5,955,425 and 6,110,892. The following linear analogue (truncated hPTH), hPTH-(1-31)-NH2, has only AC-stimulating activity and has been shown to be fully active in the restoration of bone loss in the ovariectomized rat model (Rixon, R. H. et al (1994) J. Bone Miner. Res. 9, 1179-1189; Whitfield et al (1996), Calcified Tissue Int. 58, 81-87; and Willick et al, U.S. Pat. No. 5,556,940):
Ser Val Ser Glu Ile Gln Leu Met His Asn Leu Gly Lys His Leu Asn Ser Met Glu Arg Val Glu Trp Leu Arg Lys Lys Leu Gin Asp Val-NH2(SEQ ID NO:2).
Peptide pharmaceuticals used in the prior art frequently have been administered by injection or by nasal administration. Insulin is one example of a peptide pharmaceutical frequently administered by injection. A more preferred and convenient oral administration tends to be problematic because peptide active compounds are very susceptible to degradation in the stomach and intestines. For example, while the prior art has reported an ability to achieve reproducible blood levels of salmon calcitonin and parathyroid hormone when administered orally, these levels are low. This is believed to be because these peptide hormones lack sufficient stability in the gastrointestinal tract, and tend to be poorly transported through intestinal walls into the blood. However, injection and nasal administration are significantly less convenient than, and involve more patient discomfort than, oral administration. Often this inconvenience or discomfort results in substantial patient noncompliance with a treatment regimen. Thus, there is a need in the art for more effective and reproducible oral administration of peptide pharmaceuticals like insulin, salmon calcitonin, parathyroid hormone and others discussed in more detail herein.
Proteolytic enzymes of both the stomach and intestines may degrade peptides, rendering them inactive before they can be absorbed into the bloodstream. Any amount of peptide that survives proteolytic degradation by proteases of the stomach (typically having acidic pH optima) is later confronted with proteases of the small intestine and enzymes secreted by the pancreas (typically having neutral to basic pH optima). Specific difficulties arising from the oral administration of a peptide like salmon calcitonin involve the relatively large size of the molecule, and the charge distribution it carries. This may make it more difficult for salmon calcitonin to penetrate the mucus along intestinal walls or to cross the intestinal brush border membrane into the blood.
One way to improve the effectiveness of oral administration of peptides is to protect them from proteolytic enzymes in the stomach and intestine as well as enhance their absorption from the intestine thereby enhancing their bioavailability. Improving oral effectiveness is important for several reasons. First, peptides and proteins are expensive to manufacture either by chemical synthesis or recombinant DNA technologies. Therefore, the more one increases bioavailability, the lesser the amounts that will be required in an oral formulation of a therapeutic drug.
Second, the greater the bioavailability of an oral peptide, the less the variability in the dosage absorbed by an individual on a day to day basis.
Third, the greater the bioavailability of an oral peptide, the less the concern about breakdown products of the peptide since such breakdown products can act as agonists or antagonists of the receptors where the peptide binds to elicit biological activity.